1. Field of The Invention
This invention relates to deep well drilling employing percussion drilling tools, sometimes referred to as down-hole-percussion drill motors, which are used for oil drilling and penetration of rock substrate, often to great depths in the order of 10,000 feet or more. A casing or barrel assembly is provided for housing a pressured working fluid distributor and a reciprocating piston member for hammering an anvil and bit shank piece by means of regulated air pressure, while a central passage formed by a mud rod allows simultaneous application of a flushing fluid such as mud or bentonite. Drill stem sections are specially constructed to provide intake and exhaust passages for the pressured working fluid as well as a mud passage for the flushing fluid or mud and are connected in string like fashion with additional sections added as the depth of the well increases, the working tool being connected to the lower most section of the string.
2. Description of the Prior Art
There are three common methods of drilling: rotary, pneumatic or percussion, and cable tool. Cable tool drilling has been used for hundreds of years to drill water wells and was the method used by Colonel Drake to drill the first oil well in 1859.
In cable tool drilling, a chisel-shaped bit is lowered into the hole on a steel-wire line. The bit is spudded, that is, raised and then dropped, breaking off small pieces of the formation. After a few feet have been drilled, the bit is removed and bailer is lowered into the hole on a wire line. The pieces of the formation chipped off by the bit are picked up by the bailer and removed from the hole so that drilling can continue. Satisfactory progress can be made only if water or other liquid is not allowed to remain in the hole. Any excess water must be removed, although a minimum amount can be tolerated.
There are a few advantages to cable tool drilling. One is that relatively large pieces of the formation are brought to the surface and can be identified easily as to the particular formation being drilled. Another is that some formations are sensitive to water-based fluids used in rotary drilling, and the use of cable tools reduces this problem.
The principal disadvantage of cable tools are a comparatively slow drilling rate, a practical depth limitation of less than 4,000 feet, and the lack of drilling fluid to protect against blowout.
Deep well drilling frequently requires wells which extend in excess of 10,000 feet. Drilling to these depths is usually effected by a percussion tool which progressively drills into the earth. The percussion tool is generally threadingly connected by means of a top sub assembly or adapted member (generally referred to as the top sub) to the end of a lowermost section of a drill stem. As the tool progresses into the hole additional sections of the drill stem are coupled to the other end of the last or uppermost drill stem section in string-like fashion. The drilling tool or hammer itself is generally only about forty five inches in length, while a drill stem section generally runs about 20 or 30 feet in length. Thus, hundreds of drill stem sections may be connected to form a string extending the length of the hole. Each stem includes a central passage which allows pressured working fluid to be supplied to the top sub which in turn serves to direct the pressured working fluid to the working chamber of the percussion tool or hammer.
A conventional percussion drill motor of this type is as shown and described in U.S. Pat. No. 3,503,459. In arrangements of this type, the air pressure is released from the drill bit and forced, together with the cuttings and any other debris at the bottom of the hole, including water, upward into the annular space between the drill stem and the walls of the hole and out the top. In this way the bottom of the hole is continually flushed and kept relatively clean to enable the surface to be broken up by the pneumatic action of the reciprocating bit.
With the increasing pressures that are experienced at such great depths due to subterranian water, a "flood out" condition may occur in which the water pressure at the bottom of the hole will be equal to or greater than the air pressure applied to the pneumatic hammer so that further progress is severly impeded if not curtailed. When this occurs, the drill stem must usually be tripped or pulled out of the hole and the pneumatic hammer and percussion bit are replaced with a rotatable tricone bit assembly in which the primary mode of penetration is rotary rather than reciprocal. It should be appararent that pulling the stem out of the hole and replacement of the bit, because of the severe depth of wells being drilled, requires a costly delay and effort in the drilling operation which most drill riggers try to avoid.
In rotary drilling, the bit is attached by a collar to the lowermost end of a drill pipe and lowered to the bottom of the hole. At the top of the drill pipe is a joint of square or hexagonal pipe called the kelly. The kelly passes through a rotary table on the floor of the drilling rig. Drive bushings in the rotary table clamp against the kelly and the rotary table is turned by an engine which, furnishes power to the rig. The rotation of the kelly is imparted through the drill pipe column (drill stem) to the bit at the bottom of the drill pipe to cut rock formations and thereby make a hole. Rotation speed varies from 50 to 350 revolutions per minute. Additional weight to the bit will cause drilling to proceed at a faster rate, so thick-walled sections of drill pipe known as drill collars are placed just above the drilling bit.
A rotary drilling rig has the following components: derrick, drawworks, a system of blocks and steel rope, rotary table and drive, engines, mud pumps and mud circulating system. The derrick or mast is a conventional steel frame structure placed over the well to support the drill stem and other equipment run in the hole. When drill stem is pulled from the hole, it is "racked" in the derrick until rerun. The drill stem string is suspended from the derrick on a steel line running from the drawworks over the crown block atop the derrick down to the travelling block and swivel in which the kelly turns. The drawworks is the hoisting equipment used to raise and lower pipe into the hole. The engines supply the power to drive the drawworks, rotary table, mud pumps, and electrical system around the rig. The mud system includes mud pumps, mud pits and mud lines. The drill stem string includes the drilling bit, drill collars, drill stem sections and kelly.
Pieces of rock or cuttings cut by the drill bit are removed from the bottom of the hole by drilling fluid or mud. The drilling mud is circulated by mud pumps down the center of the drill pipe, out through holes in the drill bit, and back to the surface around the outside wall of the drill pipe. As the mud returns to the surface, it carries the rock cuttings with it.